(1) Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and in particular relates to a technology for, in a fixing device that melts toner by heating a heating body in a fixing rotation body by the electromagnetic induction and fixes the melted toner onto a recording sheet, preventing non-paper-pass-through parts in the fixing rotation body from being overheated, while maintaining an appropriate fixing temperature in a paper-pass-through part.
(2) Description of the Related Art
In recent years, an electromagnetic induction heating method has been adopted in a fixing device provided in an image forming apparatus. With the electromagnetic induction heating method, a fixing rotation body (such as a fixing belt or a fixing roller) having an electromagnetic induction heating layer therein is heated by the electromagnetic induction heating with use of an exciting coil. Fixing devices adopting the electromagnetic induction heating method can directly heat the electromagnetic induction heating layer by a flux generated by the exciting coil, thus achieving a high energy efficiency and a short warming-up time.
However, the electromagnetic induction heating layer is formed to be extremely thin so that it has a low heat capacity to achieve a high energy efficiency and a quick warming-up. For this reason, the heat transfer speed in a traverse direction (an axis direction/width direction of the fixing rotation body) perpendicular to the thickness direction of the fixing rotation body is low. When recording sheets of a small size are passed through in continuation, a part of the fixing rotation body over which the recording sheets are passed through (hereinafter referred to as “paper-pass-through part”) loses heat, but a part of the fixing rotation body over which the recording sheets are not passed through (hereinafter referred to as “non-paper-pass-through part”) does not. This would lead to an overheat of the non-paper-pass-through part (to, for example, 220° C. or more).
When a recording sheet of a large size is passed through while the non-paper-pass-through part of the fixing member is overheated, a problem such as a high-temperature offset due to excessive melting of the toner or uneven gloss may occur. Also, if the fixing device is further heated, PFA (perfluoroalkoxy) tubes covering the fixing roller and a pressure roller may be melted and destroyed.
In view of this, some technologies have been developed to control the amount of heat generated in the non-paper-pass-through part. As one example of such, Japanese Patent Publication No. 4264086 discloses a technology in which a long, cylindrical center core is provided in parallel with a longitudinal direction of an exciting coil, a part of a circumferential surface of the center core corresponding to the non-paper-pass-through part is cut to provide an opening part, and when a fixing is performed on a recording sheet of a small size, the center core is rotated to a position where the opening part faces a heating roller, thereby increasing a distance between the heating roller and the part of the circumferential surface of the center core corresponding to the non-paper-pass-through part of the heating roller, weakening the magnetic link between these, and restricting the heating in the non-paper-pass-through part of the heating roller.
However, the difference in temperature or temperature distribution state between the non-paper-pass-through part and the paper-pass-through part varies depending on the size of the recording sheet passed through, or the number of recording sheets passed through in continuation. Also, when recording sheets of different sizes are passed through in continuation, these differences between the non-paper-pass-through part and the paper-pass-through part vary depending on the combination of sizes of recording sheets, the order in which recording sheets are passed through, or the number of recording sheets passed through for each size. Furthermore, the speed at which the heat transfers from the non-paper-pass-through part having a high temperature to the paper-pass-through part having a low temperature varies depending on the difference in temperature therebetween.
However, according to the structure disclosed in Japanese Patent Publication No. 4264086, the area, in which the distance between the center core and the heating roller is changed, is defined uniformly in correspondence with the non-paper-pass-through part for the currently passed-through recording sheet. With this structure, the difference in temperature distribution between the non-paper-pass-through part and the paper-pass-through part, which occurs due to the difference in the size of the recording sheet passed through, the number of recording sheets passed through, and the order in which recording sheets are passed through, cannot be dealt with flexibly and the temperature may not be controlled appropriately.